1. Technical Field
The present invention generally relates to a technical field of mobile communications, and especially relates to an adaptive control apparatus configured to adaptively control directivity of an array antenna.
2. Background Technology
In the technical field, wherein cellular communications are representative applications, the communication bandwidth is broadening as higher speed, higher quality, and the like, are required. In order to transmit a wide-band radio signal to every corner of a cell, higher power is needed than transmitting a narrow-band signal. However, since the transmitting capacity of a mobile terminal has limitations that are more severe than a radio base station, transmission power cannot be simply increased. On the other hand, if a cell radius is made smaller, although the problem of transmission power is solved, the number of radio base stations that have to be installed in an extensive service area remarkably increases, and this is not a viable solution from viewpoints such as facility investment and system management. The problem cannot be fully solved even if transmission bandwidths of uplinks and downlinks are asymmetrically arranged.
In order to cope with the problem, there is technology for maintaining the size of the cell radius by sharply forming the directivity of an antenna such that a radio wave is unevenly and adaptively distributed in a desired direction. There are two techniques in this technology, one is a beam forming method that adaptively controls a main beam of the antenna to the direction of the desired wave of choice (namely, S of SINR=S/(N+I) is increased), and the other is a null forming method that adaptively controls the null point of the antenna to the direction of an interference wave (namely, I of SINR=S/(N+I) is decreased). In either case, the adaptive control technique estimates an arrival direction (DoA: direction of arrival) of a path, and directs a strong directive beam (a beam of a predetermined narrow width) to the desired direction by sequentially updating two or more weighting factors of two or more antenna elements using convergence algorithms, such as a steepest descent method. The beam width is set up so that the received power at a cell edge is greater than a system design value.
In a communication system that uses a spread spectrum method, transmission power control (TCP) controls the transmission power of a mobile terminal so that the communication quality of a signal received by a radio base station is adjusted to the predetermined level (especially receiving level). Specifically, the transmission power of the mobile terminal is increased or decreased according to the distance between the radio base station and the mobile terminal.
FIGS. 1A and 1B give antenna beam patterns for explaining conventional beam control. In FIG. 1A, the mobile terminal located at a cell edge moves at speed v in a circumferential direction with reference to the radio base station. The mobile terminal transmits at transmission power PT1, and received power PR1 at the radio base station is a function of the transmission power PT1, a beam gain PG, and a distance r1. Angle change per unit time (angular speed) with reference to the radio base station when the mobile terminal moves is expressed by Δθ1, and power gain changes by ΔPG1=PG(0)−PG(Δθ1). Therefore, the power received by the radio base station after the mobile terminal movement becomes
                              PR          1                =                ⁢                              PT            1                    +                      PG            ⁡                          (                              Δθ                1                            )                                -                                    P              ATT                        ⁡                          (                              r                1                            )                                                              =                ⁢                              PT            1                    +                      PG            ⁡                          (              0              )                                -                      Δ            ⁢                                                  ⁢                          PG              1                                -                                    P              ATT                        ⁡                          (                              r                1                            )                                          
Here, PG(θ) represents a beam gain at a phase angle θ, and PATT(r) represents an amount of space attenuation of the radio wave corresponding to the distance r.
Further, in FIG. 1B, the case wherein a mobile terminal located near the radio base station moves in a circumferential direction at the speed v relative to the radio base station is shown. The mobile terminal transmits at transmission power PT2, the angle change per unit time (angular speed) with reference to the radio base station is Δθ2, and, the power gain changes by ΔPG2=PG(0)−PG(Δθ2). Therefore, power received by the radio base station after the mobile station movement becomes
                              PR          2                =                ⁢                              PT            2                    +                      PG            ⁡                          (                              Δθ                2                            )                                -                                    P              ATT                        ⁡                          (                              r                2                            )                                                              =                ⁢                              PT            2                    +                      PG            ⁡                          (              0              )                                -                      Δ            ⁢                                                  ⁢                          PG              2                                -                                    P              ATT                        ⁡                          (                              r                2                            )                                          
According to the conventional beam control technique, the beam is formed at a fixed beam width regardless of the distance from the radio base station to mobile terminals, and the beam direction is suitably adjusted. As seen from FIGS. 1A and 1B, for the given same moving speed v and direction (circumferential direction) of the mobile terminals, the angular speed (Δθ1) when the distance to the radio base station is great is smaller than the regular speed (Δθ2) when distance is small, that is Δθ1<Δθ2. Accordingly, as for the amounts of power gain change, ΔPG1<ΔPG2. Correspondingly, as for the power PR1 and PR2 received by the radio base station from the mobile terminals after moving, the received power PR1 from the distant mobile terminal does not become so small (decreased only by ΔPG1), and the received power PR2 from the nearby mobile terminal becomes very small (decreased by ΔPG2). For this reason, in order to maintain the level of the signal that the radio base station receives from the mobile terminal that is near and moves, higher transmission power has to be used, or alternatively, the direction of the beam must be adjusted before the angular speed becomes great.
However, since the former conventional technique is to increase the transmission power of the mobile station that is near, interference to other signals occurs, and this is not a desirable technique. According to the latter conventional technique, highly efficient adaptive control that can follow the mobile terminal at a high speed before angular speed becomes large has to be provided in order to adjust the direction of the beam, and the control burden of a system becomes great.
In this connection, Patent Reference 1 (JPA 2002-94448) discloses a technology of performing beam control per group, grouping two or more mobile stations that are close to each other into a group. There, a beam is set such that it covers the area in which the group is located. In this manner, the control burden required of the system and the base station is reduced compared with the case where a beam is controlled per mobile station.
However, grouping as above is not always possible in a communication system, and when grouping is difficult, the desired result cannot be obtained.
Patent Reference 1 JPA 2002-94448